Ventilator or extractor units of the above noted character are already known, and generally fall into two categories, in a first one of which a fan driven by an electric motor is used to draw air, fumes etc; through the units, and in the second of which categories a freely rotatable, non-driven fan is, in use, caused to rotate by the passage of air in either direction through the unit in response to an air-pressure difference between the opposite sides of the pane, panel or the like in which the unit is mounted. The third category is the louvred or squared sectional trim designed to neaten a ventilation opening. Units of the first category are fairly effective, but have the disadvantages of being a relatively heavy, expensive to manufacture, somewhat difficult to install and of course, of consuming electrical energy. Units of the second category, on the other hand, whilst being free of most of the above noted disadvantages have, in general, the serious disadvantage that they are relatively ineffective, the freely rotatable fan serving mainly to distribute the air passing in either direction through the unit somewhat so as to reduce the intensity of the draughts which would result if the unit simply provided a completely open aperture unobstructed by a fan. The third category simply provides a completely open aperture allowing unrestricted air movement in either direction. These disadvantages often result in the ventilator being immobilised or removed.